In the erection of wind turbines, the nacelle is usually first mounted on a tower, and the rotor is then connected to the nacelle. If the rotor blades are mounted individually on the hub of the rotor, the procedure is normally such that the hub of the rotor is rotated into a predefined angular position, such that the rotor blade can be brought to the hub from a predefined direction. For the mounting of the subsequent rotor blade, the hub is rotated such that the subsequent rotor blade can be brought to the hub from the same direction. If, for example, the rotor has three rotor blades, the hub is rotated by 120° between the mounting of two rotor blades.
In the intermediate stage, in which only some of the rotor blades have been attached to the hub, the rotor is highly unbalanced. Consequently, a large torque is required to change the angular position of the rotor. A device that acts directly upon the rotor shaft in order to apply this torque is described, for example, in EP 1 659 286.
If a gearbox is provided between the rotor and the generator of the wind turbine, the gear ratio of the gearbox can be used to reduce the torque to be applied. For this purpose, a turn drive is connected to the high-speed shaft of the gearbox. The torque of the turn drive acting upon the high-speed gear shaft increases according to the gear ratio of the gearbox, such that the turn drive only has to apply a comparatively small torque in order to rotate the rotor, despite the unbalance. Devices that act upon the high-speed gear shaft in order to rotate the gearbox are described, for example, in EP 1 167 754, EP 2 116 722 and DE 103 34 448.
In the case of this procedure, there arises the problem that, via the gearbox, a torque is transmitted that is close to the load limits of the gearbox. If further forces are added, for example as a result of a gust, the load limits may be exceeded. Overloading of the gearbox is to be avoided, however, in order that the gearbox does not sustain damage even before the wind turbine is put into operation for the first time.
The invention is based on the object of presenting a turn drive and a method for rotating the rotor shaft of a wind turbine that avoid overloading of the gearbox. Proceeding from the prior art, the object is achieved with the features of the independent claims. Advantageous embodiments are given in the dependent claims.